Igneous Rock Associations 28. Construction of a Venusian Greenstone Belt: A Petrological Perspective

The crustal evolution of Venus appears to be principally driven by intraplate processes that may be related to mantle upwelling as there is no physiographic (i.e. mid-ocean ridge, volcanic arc) evidence of Earth-like plate tectonics. Rocks with basaltic composition were identified at the Venera 9, 10, 13, and 14, and Vega 1 and 2 landing sites whereas the rock encountered at the Venera 8 landing site may be silicic. The Venera 14 rock is chemically indistinguishable from terrestrial olivine tholeiite but bears a strong resemblance to basalt from terrestrial Archean greenstone belts. Forward petrological modeling (i.e. fractional crystallization and partial melting) and primary melt composition calculations using the rock compositions of Venus can yield results indistinguishable from many volcanic (ultramafic, intermediate, silicic) and plutonic (tonalite, trondhjemite, granodiorite, anorthosite) rocks that typify Archean greenstone belts. Evidence of chemically precipitated (carbonate, evaporite, chert, banded-iron formation) and clastic (sandstone, shale) sedimentary rocks is scarce to absent, but their existence is dependent upon an ancient Venusian hydrosphere. Nevertheless, it appears that the volcanic–volcaniclastic–plutonic portion of terrestrial greenstone belts can be constructed from the known surface compositions of Venusian rocks and suggests that it is possible that Venus and Early Earth had parallel evolutionary tracks in the growth of proto-continental crust.

Lithosphere–asthenosphere interactions beneath northeast China and the origin of its intraplate volcanism

Northeast China hosts one of the largest Cenozoic intraplate volcanic regions in the world. However, the mechanisms that generate the volcanism, its spatial-temporal distribution, and compositional signatures remain highly debated due to the lack of high-resolution images of the mantle’s thermochemical structure. We jointly inverted new surface-wave dispersion data, surface heat flow, geoid height, and elevation data to image the fine-scale thermal and compositional structures beneath northeast China and infer regions of partial melting in the mantle. Our model reveals a complex circulation pattern in the asthenosphere and a highly variable lithospheric structure. Combining predictions from our model with independent geochemical data from recent basaltic volcanism, we demonstrate that the generation, location, and composition of intraplate volcanism in this region are controlled by the interaction between shallow asthenospheric circulation and lithospheric thickness. The modeling approach and correlations between basaltic composition and mantle state identified in our study are globally applicable to assessing mantle conditions over time in other continental regions.

Thermodynamic limits for assimilation of silicate crust in primitive magmas

Some geochemical models for basaltic and more primitive rocks suggest that their parental magmas have assimilated tens of weight percent of crustal silicate wall rock. But what are the thermodynamic limits for assimilation in primitive magmas? We pursue this question quantitatively using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems—the Magma Chamber Simulator (https://mcs.geol.ucsb.edu)—and focus on modeling assimilation of wall-rock partial melts, which is thermodynamically more efficient compared to bulk assimilation of stoped wall-rock blocks in primitive igneous systems. In the simulations, diverse komatiitic, picritic, and basaltic parental magmas assimilate progressive partial melts of preheated average lower, middle, and upper crust in amounts allowed by thermodynamics. Our results indicate that it is difficult for any subalkaline primitive magma to assimilate more than 20–30 wt% of upper or middle crust before evolving to compositions with higher SiO2 than a basaltic magma (52 wt%). On the other hand, typical komatiitic magmas have thermodynamic potential to assimilate as much as their own mass (59–102 wt%) of lower crust and retain a basaltic composition. The compositions of the parental melt and the assimilant heavily influence both how much assimilation is energetically possible in primitive magmas and the final magma composition given typical temperatures. These findings have important implications for the role of assimilation in the generation and evolution of, e.g., ultramafic to mafic trans-Moho magmatic systems, siliceous high-Mg basalts, and massif-type anorthosites.

Alkali-Activation of Synthetic Aluminosilicate Glass With Basaltic Composition

Alkali-activated materials (AAMs) are a potential alternative to Portland cement because they can have high strength, good durability and low environmental impact. This paper reports on the structural and mechanical characteristics of aluminosilicate glass with basalt-like compositions, as a feedstock for AAMs. The alkali-activation kinetics, microstructure, and mechanical performance of the alkali activated glass were investigated. The results show that AAMs prepared from basalt glass have high compressive strength (reaching up to 90 MPa after 7 days of hydration) compared to those made using granulated blast furnace slag (GBFS). In addition, calorimetry data show that the hydrolysis of the developed glass and subsequent polymerization of the reaction product occur at a faster rate compared to GBFS. Furthermore, the obtained results show that the alkali activation of the developed glass formed sodium aluminosilicate hydrate (N-A-S-H) intermixed with Ca aluminosilicate hydrate gel (C-A-S-H), while the alkali activation of GBFS resulted in predominantly C-A-S-H gel. The developed glass can be formed from carbonate-free and abundant natural resources such as basalt rocks or mixtures of silicate minerals. Therefore, the glass reported herein has high potential as a new feedstock of AAMs.

Volatile Content Implications of Increasing Explosivity of the Strombolian Eruptive Style along the Fracture Opening on the NE Villarrica Flank: Minor Eruptive Centers in the Los Nevados Group 2

Potential flank eruptions at the presently active Villarrica, Southern Andes Volcanic Zone (33.3–46 °S) require the drawing of a comprehensive scenario of eruptive style dynamics, which partially depends on the degassing process. The case we consider in this study is from the Los Nevados Subgroup 2 (LNG2) and constitutes post-glacial minor eruptive centers (MECs) of basaltic–andesitic and basaltic composition, associated with the northeastern Villarrica flank. Petrological studies of the melt inclusions volatile content in olivine determined the pre-eruptive conditions of the shallow magma feeding system (<249 Mpa saturation pressure, 927–1201 °C). The volatile saturation model on “pressure-dependent” volatile species, measured by Fourier Transform Infrared Microspectrometry (FTIR) (H2O of 0.4–3.0 wt.% and CO2 of 114–1586 ppm) and electron microprobe (EMP), revealed that fast cooling pyroclasts like vesicular scoria preserve a ~1.5 times larger amount of CO2, S, Cl, and volatile species contained in melt inclusions from primitive olivine (Fo76–86). Evidence from geological mapping and drone surveys demonstrated the eruption chronology and spatial changes in eruption style from all the local vents along a N45° corridor. The mechanism by which LNG2 is degassed plays a critical role in increasing the explosivity uphill on the Villarrica flank from volcanic vents in the NE sector (<9 km minimum saturation depth) to the SW sector (<8.1 km), where many crystalline ballistic bombs were expulsed, rather than vesicular and spatter scoria.

Thermodynamic constraints on assimilation of silicic crust by primitive magmas

&lt;p&gt;Some studies on basaltic and more primitive rocks suggest that their parental magmas have assimilated more than 50 wt.% (relative to the initial uncontaminated magma) of crustal silicate wallrock. But what are the thermodynamic limits for assimilation by primitive magmas? This question has been considered for over a century, first by N.L. Bowen and many others since then. Here we pursue this question quantitatively using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems &amp;#8212; the Magma Chamber Simulator (MCS; https://mcs.geol.ucsb.edu).&lt;/p&gt;&lt;p&gt;In the models, komatiitic, picritic, and basaltic magmas of various ages and from different tectonic settings assimilate progressive partial melts of average lower, middle, and upper crust. In order to pursue the maximum limits of assimilation constrained by phase equilibria and energetics, the mass of wallrock in the simulations was set at twice that of the initially pristine primitive magmas. In addition, the initial temperature of wallrock was set close to its solidus at a given pressure. Such conditions would approximate a rift setting with tabular chambers and high magma input causing concomitant crustal heating and steep geotherms.&lt;/p&gt;&lt;p&gt;Our results indicate that it is difficult for any primitive magma to assimilate more than 20&amp;#8722;30 wt.% of upper crust before evolving to intermediate/felsic compositions. However, if assimilant is lower crust, typical komatiitic magmas can assimilate more than their own weight (range of 59&amp;#8722;102 wt.%) and retain a basaltic composition. Even picritic magmas, more relevant to modern intraplate settings, have a thermodynamic potential to assimilate 28&amp;#8722;49 wt.% of lower crust before evolving into intermediate/felsic compositions.&lt;/p&gt;&lt;p&gt;These findings have important implications for petrogenesis of magmas. The parental melt composition and the assimilant heavily influence both how much assimilation is energetically possible in primitive magmas and the final magma composition. The fact that primitive mantle melts have potential to partially melt and assimilate significant fractions of (lower) crust may have fundamental importance for how trans-Moho magmatic systems evolve and how crustal growth is accomplished. Examples include generation of siliceous high-magnesium basalts in the Precambrian and anorogenic anorthosite-mangerite-charnockite-granite complexes with geochemical evidence of considerable geochemical overprint from (lower) crustal sources.&lt;/p&gt;

Earthquake Analysis Suggests Dyke Intrusion in 2019 Near Tarawera Volcano, New Zealand

Tarawera volcano (New Zealand) is volumetrically dominated by rhyolitic lavas and pyroclastic deposits, but the most recent event in AD 1886 was a basaltic Plinian fissure eruption. In March 2019 a swarm of at least 64 earthquakes occurred to the NE of Tarawera volcano, as recorded by the New Zealand Geohazard Monitoring Network (GeoNet). We use seismological analysis to show that this swarm was most likely caused by a dyke that intruded into the brittle crust between depths of 8–10 km and propagated toward Tarawera volcano for 2 km at a rate of 0.3–0.6 m s−1. We infer that this was a dyke of basaltic composition that was stress-guided toward Tarawera volcano by the topographic load of the volcanic edifice. Dyke intrusions of this nature are most likely a common occurrence but a similar process may have occurred during the 1886 eruption with a dyke sourced from some lateral distance away from the volcano. The 2019 intrusion was not detected by InSAR geodesy and we use synthetic models to show that geodetic monitoring could only detect a ≥6 m wide dyke at these depths. Improvements to geodetic monitoring, combined with detailed seismological analysis, could better detect future magmatic intrusions in the region and serve to help assess ongoing changes in the magmatic system and the associated possibilities of a volcanic event.

Distinct chemical and mineralogical composition of Icelandic dust compared to northern African and Asian dust

Iceland is a highly active source of natural dust. Icelandic dust has the potential to directly affect the climate via dust–radiation interaction and indirectly via dust–cloud interaction, the snow/ice albedo effect and impacts on biogeochemical cycles. The impacts of Icelandic dust depend on its mineralogical and chemical composition. However, a lack of data has prevented an accurate assessment of the role of Icelandic dust in the Earth system. Here, we collected surface sediment samples from five major Icelandic dust hotspots. Dust aerosols were generated and suspended in atmospheric chambers, and PM10 and PM20 fractions were collected for further analysis. We found that the dust samples primarily consist of amorphous basaltic materials ranging from 8 wt % (from the Hagavatn hotspot) to 60 wt %–90 wt % (other hotspots). Samples had relatively high total Fe content (10 wt %–13 wt %). Sequential extraction of Fe to determine its chemical form shows that dithionite Fe (Fe oxides such as hematite and goethite) and ascorbate Fe (amorphous Fe) contribute respectively 1 %–6 % and 0.3 %–1.4 % to the total Fe in Icelandic dust. The magnetite fraction is 7 %–15 % of total Fe and 1 %–2 wt % of PM10, which is orders of magnitude higher than in mineral dust from northern Africa. Nevertheless, about 80 %–90% of the Fe is contained in pyroxene and amorphous glass. The initial Fe solubility (ammonium acetate extraction at pH 4.7) is from 0.08 % to 0.6 %, which is comparable to low-latitude dust such as that from northern Africa. The Fe solubility at low pH (i.e. pH 2) is significantly higher than typical low-latitude dust (up to 30 % at pH 2 after 72 h). Our results revealed the fundamental differences in composition and mineralogy of Icelandic dust from low-latitude dust. We attribute these differences to the low degree of chemical weathering, the basaltic composition of the parent sediments and glacial processes. Icelandic dust contributes to the atmospheric deposition of soluble Fe and can impact primary productivity in the North Atlantic Ocean. The distinct chemical and mineralogical composition, particularly the high magnetite content (1 wt %–2 wt %), indicates a potentially significant impact of Icelandic dust on the radiation balance in the subpolar and polar regions.

Spectral characterisation of 14 V-type candidate asteroids from the MOVIS catalogue

Most of the currently known basaltic (V-type) asteroids are believed to be past or present members of the Vesta dynamical family. The rising discoveries of V-type asteroids that are not linked to the Vesta family dynamically suggest that a number of major basaltic bodies may have been present during the early stages of the solar system. Using the near-infrared (NIR) colour data in the Moving Objects from VISTA Survey (MOVIS) catalogue, a list of 477 V-type candidates was compiled, with more than half of them outside the Vesta family. In this work, we aim to provide a spectral analysis of 14 V-type candidates of various dynamical types. The computed visible and NIR spectral parameters are used to investigate evidence of space-weathering or mineralogical differences from the expected basaltic composition. Based on the analysis of their visible spectra, we confirm 11 new V-type asteroids: six low-i asteroids – (3188) Jekabsons, (3331) Kvistaberg, (4693) Drummond, (7223) Dolgorukij, (9007) James Bond, and (29733) 1999 BA4; along with four inner-other asteroids – (5524) Lecacheux, (19983) 1990 DW, (51742) 2001 KE55, and (90023) 2003 BD13; as well as one fugitive – (2275) Cuitlahuac. Additionally, we analysed three peculiar outer main belt V-type candidates based on their visible + NIR spectra. We confirm the diogenite-like composition of (2452) Lyot. The spectrum of asteroid (7302) is not consistent with a basaltic composition and likely reflects an S-type body. The spectrum of (14390) 1990 QP10 is similar to a V-type but it shows unique spectral features that suggest a peculiar composition. Overall, our results demonstrate the efficiency of the MOVIS catalogue in identifying V-type objects, with a success rate of over 85%. The identification of V-types in the inner main-belt is more likely due to the presence of the Vesta family and other nearby asteroids that had escaped from the family. In the middle and outer main belt, where the amount of data is more limited, the proportion of false positives increases.

Emplacement and Evolution of The Nlonako Ring Complex in The Southern Domain of The Cameroon Line

The Cameroon Line (CL) appears as a SW-NE straight line characterized by an intense volcanic activity of basaltic composition with alkaline plutonic complexes, including the Nlonako ring complex (NRC) emplaced in the southern part of the CL. Many hypotheses have been proposed to explain the origin and petrogenetic evolution of these complexes but very few of these works focus or attempt to propose their structural emplacement model, though structural data recorded by them can permit such studies. Using petrographic, structural data and space images, we propose an emplacement model of the NRC and other ring complexes along the CL. The NRC is a ring complex of 10 km diameter slightly elongated in the NNE-SSW direction mainly composed of plutonites among which syenites, gabbros, diorites and biotite-amphibole granites, and a few volcanites made up of rhyolites and basalts occurring as veins or boulders in syenites. This complex was emplaced as sill intrusive body within Pan-African k-feldspars megacrysts granites and gneiss host rocks under fractures control. These fractures developed as result of stress release consecutive to the readjustment of the crust during and after the late Cretaceous general extension, therefore facilitating the upwelling of the mantle plume and the generation of magma that vertically uplift. The progressive magmatic pressure decreases after the NRC emplacement in addition to conjugated fractures networks developed at superficial crust level finally lead to the cauldron subsidence of the NRC summit. This subsidence was facilitated by the downward sliding of rocks along the WNW-ESE fault, finally leading to the formation of a caldera at the summit of the NRC. The NRC and other anorogenic complexes aligned along the CL are located in a tension gash form by the Cretaceous sinistral activation of the N70E Adamawa fault. This left lateral wrench movements developed a tension gash, overprinted on pre-existing transcurrent mega-faults with as result, the development of "en-echelon" fractures. Stress release during the late Cretaceous general extensional phase probably accelerated the uplift of magmas and emplacement of ring complexes along the "en-echelon" regional fractures in an extensional setting during the Tertiary (60 - 30 Ma). This therefore explained the alignment of anorogenic complexes along the N30E CL corridor that are highly correlated to lineament networks and Pan-African megafaults